Reactive choke for automatic wellbore fluid management and methods of using same

ABSTRACT

Fluid actuated chokes include an expandable material that expands due to an undesired fluid coming in contact with the choke. The choke includes one or more passageways through which desired fluids from a well flow unimpeded. The choke is disposed in a downhole tool as part of a downhole completion. When one or more undesired fluids enters a production stream flowing through the downhole completion and, thus, the downhole tool, and contacts the choke, the expandable material expands causing fluid flow through the passageway(s) to be restricted and, in some cases, completely closed off. Thus, the choke automatically detects and reacts, i.e., restricts fluid flow through the choke, when contacted by the undesired fluid(s).

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No. 13/298,530, filed Nov. 17, 2011, and entitled “Reactive Choke for Automatic Wellbore Fluid Management and Methods of Using Same” the disclosure of which is hereby incorporated by reference in its entirety.

BACKGROUND

1. Field of Invention

The present invention is directed to chokes for use in downhole tools, and in particular, to reactive, fluid actuated, chokes disposed in downhole tools that enable automatic prevention of undesired fluids from entering or exiting a production stream in downhole completions by expanding upon exposure to one or more undesired fluids causing restriction of fluid flow through the choke and, thus, the downhole tools.

2. Description of Art

During production of fluids from a well, one or more fluid flows from the formation of the well into a downhole completion. This flowing of fluid is referred to as a production stream. The terms “fluid” and “fluids” as used herein can include oil, gas, water, brine, and the like. Generally, it is desired to produce only hydrocarbons from a well and leave all other fluids within the well. However, in some instances, it may be desirable to remove well or brine from the well and leave the hydrocarbons for later production. In either situation, at least one fluid is desired to be produced, i.e., flowed from the formation, into the downhole completion and out of the well, while other fluids are undesired.

SUMMARY OF INVENTION

Broadly, the fluid actuated chokes in downhole tools disclosed herein include an expandable body comprising one or more passageways disposed therein. Desired fluid(s) are permitted to flow through the passageway(s) unimpeded as part of the production stream. Undesired fluid(s) are restricted from flowing through the passageways due to expansion of the expandable body. The expandable body expands when contacted with the undesirable fluid(s). As a result, the passageway(s) move from an initial position which provides a initial flow rate through the passageway(s) towards an expanded or restricted position that provides a second, lesser, flow rate through the passageway(s).

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of one specific embodiment of a downhole tool having one specific embodiment of the fluid actuated chokes disclosed herein.

FIG. 2 is a perspective view of one specific embodiment of the fluid actuated chokes disclosed herein.

FIG. 3A is a partial cross-sectional view of the reactive choke of FIG. 2 taken along line 2-2 showing the fluid actuated choke in its initial position.

FIG. 3B is a partial cross-sectional view of the reactive choke of FIG. 2 taken along line 2-2 showing fluid actuated choke in one of its plurality of restricted or expanded positions.

FIG. 4 is a partial cross-sectional view of another specific embodiment of the fluid actuated chokes disclosed herein showing the fluid actuated choke in its initial position.

FIG. 5 is a partial cross-sectional view of an additional specific embodiment of the fluid actuated chokes disclosed herein showing the fluid actuated choke in its initial position.

FIG. 6 is a partial cross-sectional view of another specific embodiment of the fluid. actuated chokes disclosed herein showing the fluid actuated choke in its initial position.

FIG. 7 is a partial cross-sectional view of an additional specific embodiment of the fluid actuated chokes disclosed herein showing the fluid actuated choke in its initial position.

FIG. 8 is a cross-sectional view of another embodiment of a downhole tool having one specific embodiment of the fluid actuated chokes disclosed herein.

FIG. 9 is a partial cross-sectional view of the downhole tool of FIG. 8 showing the fluid actuated choke in its initial position.

While the invention will be described in connection with the preferred embodiments, it will be understood that it is not intended to limit the invention to these embodiments. On the contrary, it is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF INVENTION

Referring now to FIGS. 1-3B, downhole tool 20 comprises tubular member 21, having outer wall surface 22 and inner wall surface 23 defining bore 24. Choke 30 is disposed within bore 24 thereby dividing bore 24 into upper bore portion 25 and lower bore portion 26. As discussed in greater detail below, a fluid, such as oil, gas, brine, water and the like, initially flow through choke 30 either in an upward direction or a downward direction. In other words, the fluid initially flows from lower bore portion 26, through choke 30, and into upper bore portion 25, or from upper bore portion 25, through choke 30, and into lower bore portion 26.

As best illustrated in FIG. 2, choke 30 comprises body 31, having upper surface 32, lower surface 33, side surface 34, and a plurality of passageways 35 disposed between upper and lower surfaces 32, 33 thereby permitting a fluid to flow through body 31. As shown in the FIG. 2, body 31 has one side surface 34 because body is cylindrically, or disc, shaped. In other embodiments, body 31 may have another shape, e.g., a polygonal shape such as a square shape, a rectangular shape, hexagonal shape or the like (not shown). In such embodiments, body 31 would be have more than one side surface 34.

Body 31 is formed, at least in part, by an expandable material that is capable of expanding to restrict fluid flow through passageway(s) 35 due to contact with an undesired fluid, e.g., hydrocarbon, brine, water, and the like. Referring to FIGS. 3A and 3B, when contacted by the undesired fluid, body 31 expands such that body 31 and, thus, expandable material and passageway 35, moves from the initial configuration or position (FIG. 3A) to the restricted or expanded configuration or position (FIG. 3B) in which the flow of the undesired fluid through passageway 35 is lessened. In other words, the flow rate of fluid through passageway 35 is decreased. As shown in FIG. 3B, passageway 35 is not completely closed off due to the expansion of expandable material of body 31 from the initial configuration. It is to be understood, however, that in other embodiments, passageway 35 is completely closed off such that the flow rate of fluid through passageway 35 is substantially zero, i.e., less than 5% of the original flow capacity permitted to flow through passageway 35 when the expandable material is in the initial configuration.

In one specific embodiment, body 31 is formed completely out of the expandable material. In other embodiments, body 31 includes non-expandable components such as stifling rings or other support structures or substrates to which the expandable material is connected. Further, in one embodiment, the expandable materials expand by absorbing the undesired fluid.

Suitable expandable materials include urethane and polyurethane materials, including polyurethane foams, biopolymers, and superabsorbent polymers. Nitriles and polymers sold as 1064 EPDM from Rubber Engineering in Salt Lake City, Utah are acceptable expandable materials. In one embodiment, the expandable material comprises a swellable polymer such as cross-linked or partially cross-linked polyacrylamide, polyurethane, ethylene propylene, or other material capable of absorbing hydrocarbon or aqueous, or other fluids, and, thus, swelling to restrict passageway(s) 35. Additional suitable expandable materials include elastomers such as restrict rubber (“NBR”), hydrogenated nitrile rubber (“HNBR”), carboxyl nitrile rubber (“XNBR”), silicone rubber, ethylene-propylene-diene copolymer (“EPDM”), fluoroelastomer (“FKM,” “FEPM”) and perfluoroelastomer (“FFKM”); and cross-linked polymers such as water-soluble methylcellulose, cellulose acetate phtalate, and hydroxypropyl methylcellulose polymers, poly (ethylene oxide) polymers, guar and its derivatives, polyacrylamide, silicon-based materials, and flouro-silicone based materials. Still other expandable materials are disclosed in U.S. Pat. No. 7,901,771 B2 which is hereby incorporated by reference herein in its entirety.

In another embodiment, the expandable material is a shape-memory material, for example, a compressed elastomer or polymer that is held in the compressed state by a dissolvable material. In one such embodiment, the expandable materials or body 31 itself, including the surface area of body 31 within passageway(s) 35 may be encapsulated with a layer of material dissolvable by fluids such as water, brine, hydraulic fluid, hydrocarbons, and the like. As used herein, the term “encapsulated” and “encapsulating” means that the dissolvable material forms an initial barrier between the fluid and the expandable materials or body 31. In such embodiments, the encapsulated layer allows the use of expandable materials, and body 31 formed from expandable material(s), that expand virtually instantaneously upon contacting the fluid by protecting the expandable material(s) until expansion is desired.

Encapsulating dissolvable materials for encapsulating the expandable materials may be any material known to persons of ordinary skill in the art that can be dissolved, degraded, or disintegrated over an amount of time by a temperature or fluid such as water-based drilling fluids, hydrocarbon-based drilling fluids, or natural gas. Preferably, the encapsulating dissolvable material is calibrated such that the amount of time necessary for the dissolvable material to dissolve is known or easily determinable without undue experimentation. Suitable encapsulating dissolvable materials include polymers and biodegradable polymers, for example, polyvinyl-alcohol based polymers such as the polymer HYDROCENE™ available from Idroplax, S.r.l, located in Altopascia, Italy, polylactide (“PLA”) polymer 4060D from Nature-Works™, a division of Cargill Dow LLC; TLF-6267 polyglycolic acid (“PGA”) from DuPont Specialty Chemicals; polycaprolactams and mixtures of PLA and PGA; solid acids, such as sulfamic acid, trichloroacetic acid, and citric acid, held together with a wax or other suitable binder material; polyethylene homopolymers and paraffin waxes; polyalkylene oxides, such as polyethylene oxides, and polyalkylene glycols, such as polyethylene glycols. These polymers may be preferred where water is the undesired fluid because they are slowly soluble in water.

As shown in FIG. 1, choke 30 is encircled by rigid tubular member 70 which can facilitate securing choke 30 within bore 24. Tubular member 70 provides resistance to outward expansion of body 31 and, thus, facilitates restriction of passageway(s) 35 during expansion of body 31. In the embodiment of FIG. 1, tubular member 70 also comprises a top retainer 71 disposed on upper surface 32 and a bottom retainer 72 disposed on lower surface 33 to provide resistance to upward and downward expansion of body 31. Thus, upper and lower retainers 71, 72 further facilitate restriction of passageway(s) 35. In another embodiment (not shown), upper and lower retainers 71, 72 extend over the entire upper and lower surfaces 32, 33 and include holes that are in at least partial alignment with each passageway 35. In this embodiment, upper and lower retainers 71, 72 provide addition resistance to upward and downward expansion by body 31 so that each passageway 35 can be restricted; yet fluid can flow through upper and lower retainers 71, 72 prior to expansion of body 31 due to the holes being at least partially aligned with corresponding passageways 35.

As shown in the embodiment of FIGS. 1-3B, choke 30 comprises a plurality of passageways 35 having a cylindrical cross-sectional shape such that the intersection of each passageway 35 with upper and lower surfaces 32, 33 provides a substantially circular shape, each circular shape having a substantially identical circumference. In other embodiments, such as the embodiment shown in FIG. 4, one or more passageways 35 comprises a serrated cross-sectional shape. As shown in FIG. 4, one side of the cross-sectional shape of passageway 35 has serrations that are reciprocal in shape to the serrations on the opposite side of the cross-sectional shape of passageway 35. Thus, upon expansion of body 31, the serrations of one side fit into the reciprocally-shaped serrations of the opposite side to facilitate restriction of fluid flow through passageway(s) 35.

In still other embodiments, one or more of passageways 35 may have a conically-shaped cross-section (FIG. 5) in which the intersection of passageway 35 with the upper surface 32 provides a circular shape having a circumference that is smaller than a circular shape of the intersection of passageway 35 with the lower surface. Alternatively, the circumference of the circular shape of the intersection of passageway 35 with lower surface 33 may be smaller than the circumference of the circular shape of the intersection of passageway 35 with the upper surface.

In another embodiment shown in FIG. 6, one or more passageways 35 comprises a curved cross-section, wherein one side has a convex shape and an opposite side has reciprocally shaped concave shape so that, upon expansion of body 31, the convex shaped side fits into the concave shape side to facilitate restriction of the flow of fluid through passageway(s) 35.

In yet another embodiment shown in FIG. 7, the sides of the cross-sectional shape of one or more passageways 35 have a single chevron cross-sectional shape in which one side fits into a reciprocally shaped chevron shape on an opposite side of the cross-sectional shape of passageway 35 to facilitate restriction of fluid flow through passageway(s) 35.

Referring now to FIGS. 8-9, in another embodiment, downhole tool 80 comprises tabular member 81 having outer wall surface 82 and inner wall surface 83 defining bore 84. Choke 30, upper porous media 50, and lower porous media 60, are disposed within bore 84 thereby dividing bore 84 into upper bore portion 85 and lower bore portion 86. Choke 30 can be any of the embodiments disclosed and taught herein. Upper and lower porous media 50, 60 can be any porous media known in the art that permits fluid to flow through them. Suitable porous media include Teflon foam or metal screen.

A fluid, such as oil, gas, brine, water and the like, initially flows through choke 30, upper media 50, and lower media 60 in either in an upward direction or a downward direction. In other words, the fluid initially flows from lower bore portion 86, through lower media 60, choke 30, and upper media 50 into upper bore portion 85, or from upper bore portion 85, through upper media 50, choke 30, and lower media 60 into lower bore portion 86.

As best illustrated in FIG. 9, choke 30, upper media 50, and lower media 60 are engaged to inner wall surface 83 of tubular member 85 and retained in place by upper and lower retainers 87, 89. Thus, tubular member 85 provides resistance to outward expansion of body 31 and, thus, facilitates inward expansion of body 31, thereby causing passageway(s) 35 to be restricted. Upper media 50 is engaged with upper surface 32 of body 31 and lower media 60 is engaged with lower surface 33. Thus, upper and lower media 50, 60 provide resistance to upward and downward expansion, respectively, of body 31 and, accordingly, facilitates restriction of passageway(s) 35 during expansion of body 31. In addition, upper and lower retainers 87, 89 provide additional resistance to upward and lower expansion, respectively, by body 31 to further facilitate restriction of passageway(s) 35.

In yet another embodiment, not shown in the Figures, the choke includes an opening through which a mandrel, pipe, or other tubular member is passed. In this embodiment, the choke is disposed on an outer diameter of the mandrel, pipe or other tubular member so that the fluid flows through the passageways disposed outside of the mandrel, pipe, or other tabular member. In one particular embodiment of this arrangement, the choke is static and disposed on the outer diameter of the mandrel, pipe, or other tubular member. In another particular embodiment of this arrangement, the choke is disposed on a sliding sleeve that is in sliding engagement with the outer diameter of the mandrel, pipe or other tubular member.

In operation, a downhole tool is disposed within a downhole completion at a desired location. Fluid is then permitted to flow from the formation as part of a production stream flowing through the downhole completion and, thus, through the downhole tool. Disposed within a production stream flow path through the downhole tool is a fluid activated choke such as those disclosed herein. Desired fluids are permitted to flow through the choke unimpeded. However, if an undesired fluid contacts the choke, the choke automatically restricts fluid flow through the choke due to the expansion of one or more expandable materials forming the body of the choke.

Depending on the fluid that is desired to be removed from the well, the desired fluid that is permitted to flow unimpeded through the choke can be hydrocarbons, brine, water, and the like. Similarly, the undesired fluid can also be hydrocarbons, brine, water, and the like. In other words, in some operations, it may be desirable to remove water from the well and leave hydrocarbons within the well for future production. In these operations, the choke will permit water to flow through the downhole completion unimpeded, but will automatically restrict the flow of hydrocarbons through the choke when the choke is contacted with the hydrocarbons. Conversely, in other operations it may be desirable to remove hydrocarbons from the well and leave water within the well. In these operations, the choke will permit hydrocarbons to flow through the downhole completion unimpeded, but will automatically restrict the flow of water through the choke when the choke is contacted with the water.

In certain particular embodiments of the operation of the chokes disclosed herein, the choke is reversible. That is, the choke can be closed off or restricted by contact with the undesired fluid; however, after the undesired fluid is not longer in contact with the choke, or the choke is placed in contact with a desired fluid, the passageway(s) through the choke move toward their original open or unrestricted position and, in some embodiments, the passageway(s) return all the way to their original open or unrestricted position. Thus, the desired fluid is again permitted to flow through the choke, Afterwards, the choke can again be activated by an undesired fluid to restrict fluid flow through the choke. Later, the choke can be reopened and the process of restricting fluid flow through the passageways, and then reopening the passageways, can be repeated.

In one experiment, a choke was formed comprising water swellable rubber having a blend of NBR and polyacrylamide sold under the designation DPNT04 0207 available from BASF located in Florham Park, N.J. The choke comprised a disc-shape having a continuous thickness of 0.085 inches and a diameter of 0.950 inches. Forty-Three circular holes or passageways each having a diameter of approximately 0.620 inches were disposed through the body of the choke. The choke was placed between an upper and lower porous media each comprising Teflon foam. The upper porous media had a disc-shape with a diameter of 0.950 inches and a thickness of 1.000 inch. The lower porous media had a disc-shape with a diameter of 0.950 inches and a thickness of 1.500 inches.

The upper porous media, choke, and lower porous media were placed in a 1 inch diameter flow loop with the direction of fluid flow passing through the lower porous media, then through the choke, and then through the upper porous media. Initially, oil (LVT 200) was flowed through the flow loop at 180° F. at a rate of 100 ml/min. Pressure readings, in pressure per square inch, were taken each minute for 20 seconds. The pressure remained steady at approximately 0.5 psi for the first 75 minutes of the experiment. At the 75th. minute of the experiment, the oil was replaced with a solution of 30% brine water (solution of 30% salt in water). Pressure readings were again taken each minute for 20 seconds over a 75 minute interval, starting at the 88th minute of the experiment. In addition, after 30 minutes, the percentage of brine cut was increased until the brine water reached 100%. During the 75 minute interval in which brine water was flowed through the flow loop, the pressure of the fluid flowing through the flow loop increased from approximately 1.0 psi at 30% brine water to approximately 2.9 psi at 100% brine water. At the 163rd minute of the experiment, the 100% brine water was replaced with oil (LVT 200). Thereafter from the 163rd minute through the 295th minute of the experiment pressure readings of the oil flowing through the flow loop were taken each minute for 20 seconds. The pressure of the fluid decreased from approximately 2.2 psi at the 165th (100% oil) minute of the experiment to approximately 0.9 psi at the 295th minute of the experiment.

As illustrated by the this experiment, the desired fluid, oil, is permitted to flow through the choke at a relatively low pressure; however, upon being contacted by brine water (the undesired fluid), the choke swells and the passageways are closed off causing an increase in pressure within the flow loop. Removal of the brine water reverses the swelling of the choke resulting in the passageways be reopened to permit oil to flow through. Thus, the choke is reversible and repeatable such that fluid flow through the choke can be decreased and then increased.

It is to be understood that the invention is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. For example, the passageways through the choke can have any desired or necessary cross-sectional shape to facilitate restriction of fluid flow through the choke. Further, not all of the passageways in the choke are required to have the same cross-sectional shape. Moreover, the choke can have as few as a single passageway or a plurality of passageways. Additionally, it is not required that all fluid flow be prevented from flowing through the choke after expansion of the expandable material. To the contrary, some fluid may be allowed to continue to flow through the choke which can still indicate to the operator of the well such as by pressure changes or flow rate changes that an undesired fluid has entered the production stream so that the operator can make desired adjustments to the production of the well. In addition, it is to be understood that when one of the expandable material, the body, or the passageway is in a first or an initial configuration or position, the remainder of these components is also in the first or initial configuration or position. Likewise, one of the expandable material, the body, or the passageway is in a second configuration or position, the remainder of these components is also in the second configuration or position. Further, the tubular members are not required to have a circular cross-section. Instead, the tubular member can have a polygonal shape or any other shape desired or necessary to flow a production stream through the tubular member. Moreover, the choke is not required to be cylindrically or disc shaped, but can have any other shape desired or necessary to sufficiently restrict fluid flow through the downhole tool when contacted by one or more undesired fluids. Further, in embodiments in which the choke is reversible, the passageway(s) are not required to return to their original positions. All that is required is that the passageway(s) move toward their original positions such that increased fluid flow is permitted through the passageway(s). Accordingly, the invention is therefore to be limited only by the scope of the appended claims. 

What is claimed is:
 1. A downhole tool through which a production stream flows, the downhole tool comprising: a fluid actuated choke, the fluid activated choke comprising a body having an upper surface, a lower surface and, a passageway connecting the upper surface with the lower surface, the body being comprised of an expandable material having a first configuration in which fluid flows through the passageway at a first flow rate and a second configuration in which fluid flows through the passageway at a second flow rate, the first flow rate being greater than the second flow rate, wherein the expandable material expands from the first configuration to the second configuration due to a first fluid contacting the expandable material; wherein the passageway intersects the upper surface at a first intersection, the first intersection having a first substantially circular shape; wherein the passageway intersects the lower surface at a second intersection, the second intersection having a second substantially circular shape; and wherein the first substantially circular shape comprises a first circumference and the second substantially circular shape comprises a second circumference, the first circumference being greater than the second circumference.
 2. The downhole tool of claim 1, the first fluid being in a production stream produced from a formation and wherein the expandable material moves from the second configuration to the first configuration due to a second fluid contacting the expandable material, the second fluid being in the production stream produced from the formation.
 3. A downhole tool through which a production stream flows, the downhole tool comprising: a fluid actuated choke, the fluid activated choke comprising a body having an upper surface, a lower surface and, a passageway connecting the upper surface with the lower surface, the body being comprised of an expandable material having a first configuration in which fluid flows through the passageway at a first flow rate and a second configuration in which fluid flows through the passageway at a second flow rate, the first flow rate being greater than the second flow rate, wherein the expandable material expands from the first configuration to the second configuration due to a first fluid contacting the expandable material.
 4. The downhole tool of claim 3, wherein the expandable material comprises an encapsulating dissolvable material encapsulating the expandable material to prevent expansion of the expandable material from the first configuration to the second configuration until the encapsulating dissolvable material is dissolved by the first fluid.
 5. The downhole tool of claim 4, the first fluid being in a production stream produced from a formation and wherein the expandable material moves from the second configuration to the first configuration due to a second fluid contacting the expandable material, the second fluid being in the production stream produced from the formation.
 6. The downhole tool of claim 3, wherein the upper surface of the body is in contact with a first rigid porous media.
 7. The downhole tool of claim 6, wherein the lower surface of the body is in contact with a second rigid porous media.
 8. The downhole tool of claim 7, the first fluid being in a production stream produced from a formation and wherein the expandable material moves from the second configuration to the first configuration due to a second fluid contacting the expandable material, the second fluid being in the production stream produced from the formation.
 9. A method of restricting fluid flow through a downhole tool, the method comprising the steps of: (a) flowing fluid through a fluid flow path of a downhole tool, the downhole tool having disposed within the fluid flow path a fluid actuated choke, the fluid actuated choke comprising a body having an upper surface, a lower surface and, a passageway connecting the upper surface with the lower surface, the body being comprised of an expandable material having a first configuration in which fluid flows through the passageway at a first flow rate and a second configuration in which fluid flows through the passageway at a second flow rate, the first flow rate being greater than the second flow rate; and (b) contacting the expandable material of the fluid actuated choke with a first fluid causing the expandable material to expand from the first configuration toward the second configuration causing the passageway to move from an initial position toward a closed position, thereby restricting the fluid flowing through the passageway from the first flow rate to the second flow rate. (c) contracting the expandable material of the fluid actuated choke with a second fluid causing the expandable material to move from the second configuration toward the first configuration causing the passageway to move toward the initial position, thereby increasing fluid flow through the passageway, the second fluid being in the production stream.
 10. The method of claim 9, wherein the during step (b) the first fluid dissolves a dissolvable material encapsulating the expandable material.
 11. The method of claim 10, further comprising, the fluid flowing through the fluid flow path being a production stream produced from a formation and the first fluid bring in the production stream.
 12. The method of claim 11 further comprising the step of (c) contracting the expandable material of the fluid actuated choke with a second fluid causing the expandable material to move from the second configuration toward the first configuration causing the passageway to move toward the initial position, thereby increasing fluid flow through the passageway, the second fluid being in the production stream.
 13. The method of claim 12, wherein step (b) is repeated.
 14. The method of claim 9, wherein during step (a), the fluid flows at the first flow rate through a first porous media, then through the passageway, and then through a second porous media.
 15. The method of claim 14, further comprising, the fluid flowing through the fluid flow path being a production stream produced from a formation and the first fluid bring in the production stream.
 16. The method of claim 15 further comprising the step of (c) contracting the expandable material of the fluid actuated choke with a second fluid causing the expandable material to move from the second configuration toward the first configuration causing the passageway to move toward the initial position, thereby increasing fluid flow through the passageway, the second fluid being in the production stream.
 17. The method of claim 16, wherein step (b) is repeated. 